1. Field of the Invention
This invention relates to a linear scale measuring device for electronically and optically reading the amount of linear movement of a slide table of, for example, a machine tool or the like by means of optical lattices, and more particularly to a connecting structure between a stationary link of an optical detecting head section of the measuring device and a slider having an index scale.
2. Description of the Prior Art
In order to improve working accuracy of a machine tool or the like, a linear scale measuring device using moire fringes which is schematically shown in FIGS. 3 and 4 is used on the machine tool. FIG. 3 shows a fragmentary perspective view of the machine tool having the linear scale measuring device 1 installed on a bed 3 and a slide table 4 of the machine tool. The measuring device 1 measures the amount of movement of the slide table 4 with respect to the bed 3 so that work on the slide table 4 during working operation and the like may be precisely positioned.
The slide table 4 of the machine tool 2 is engaged with a guide means provided on the bed 3 fixed on a floor and is linearly moved with respect to the bed 3. On one side surface of the slide table 4 is fixed a transparent main scale 6 by means of a mounting frame 5. The main scale 6 is formed into an elongated rectangular shape and arranged in such a manner that a longitudinal axis thereof is substantially parallel to a direction of movement of the slide table 4 and a surface thereof is parallel to the side surface of the slide table 4. A surface of the main scale 6 is formed thereon with vertical fine optical lattices 7 which have a diameter of, for example, about 10-20 .mu.m and arranged at intervals of, for example, about 10-20 .mu.m. The bed 3 is provided with a detecting head 8, which is fixed on the bed 3 and arranged opposite to the main scale 6. On a portion of the detecting head 8 facing the main scale 6, a stationary link 9 is mounted.
FIGS. 4a and 4b each show a relationship between the main scale 6 and a slider 14. As shown in FIGS. 4a and 4b, the stationary link 9 is provided with projections 10 and 11 on both ends thereof in the longitudinal direction of the main scale 6. As shown in FIG. 4(b), a tension bar 12 is mounted on the projection 11, is extended in parallel to the main scale 6 toward the projection 10 and has a free end terminated at a substantially central position between both projections 10 and 11. On the free end of the tension bar 12 is fixed a ball-and-socket joint 13. The slider 14 is connected to the stationary link 9 by means of the tension bar 12 having the ball-and-socket joint. The slider 14 is formed into a substantially rectangular shape and is provided with a transparent index scale having optical lattices, of the same construction as the optical lattices 7 of the index scale 6, and which are vertically arranged thereon. Also, there is provided a circular mounting hole 15 having a boss 15a at a side of the slider 14 opposite to the stationary link 9 and further having a substantially central portion thereof, in which the ball-and-socket joint 13 of the tension bar 12 is fitted. Thus, the slider 14 is connected to the stationary link 9 so that it may be pivotally moved about the ball-and-socket joint 13 with a certain degree of freedom. When the device 1 is assembled, the slider 14 is forced against the main scale 6 due to the elastic force of the tension bar 12, and is movably contacted with the main scale 6 by means of a bearing (not shown). Also, the optical lattices of the index scale are inclined at a micro-angle with respect to the optical lattices 7 of the main scale 6 and both scales face each other with a gap of a predetermined micro-distance. Further, the detecting head 8 is provided with a light source (not shown) and a light receptor (not shown) opposite to each other with both scales interposed therebetween. Reference numeral 16 designates a counter which is connected to the light receptor of the detecting head 8 to display the amount of movement of the slide table 4.
In the conventional linear scale measuring device 1 constructed as explained hereinabove, when the optical lattices 7 of the main scale 6 are superposed on those of the index scale at a micro-angle, a fringe pattern called moire fringes is produced on the scales. Intervals between the moire fringes thus formed may be adjusted depending on an angle between the lattices of both scales. Also, when both scales are moved with respect to each other in the direction perpendicular to that of the lattices, the moire fringes are moved in the longitudinal direction of the lattices perpendicular to that of the relative movement. Then, the light source of the detecting head 8 is turned on to emit light, which is received by the light receptor passing through the main scale and index scale.
Subsequently, when the slide table 4 is moved, the main scale 6 fixed on the slide table 4 slides in relation to the slider 14 of the detecting head 8 fixed on the bed 3, and the moire fringes are moved in order depending on a the amount of relative movement between both scales. Thus, the light from the light source is intermittently intercepted by the moire fringes, and the light receptor concerts movement of the moire fringes into an electrical signal. Then, an output of the receptor thus obtained is then subjected to a suitable processing such as integration or the like by the counter 16 so that a the amount of movement of the slide table 4 or a the amount of movement of a work (not shown) on the slide table 4 may be displayed on a display section of the counter 16.
As explained hereinabove, the conventional linear scale measuring device 1 is contructed in such a manner that the slider 14 is mounted on the free end of the single tension bar 12 provided at the stationary link 9 so as to be forcedly abutted against the main scale 6 due to the elastic force of the tension bar 12. Thus construction causes the force of the tension bar 12 to be applied to only one portion of the slider 14, to thereby deteriorate the balance of the force applied to the slider to a degree sufficient to render the slider unstable with respect to the main scale 6. This results in the slider 14 being pivotally moved about the circular hole 15 and ball-and-socket joint 13 which serves as a connection means between the tension bar 12 and the slider 14, and a relative position between both optical lattices is varied leading to a failure in measurement.
Also, the conventional device is disadvantageous in that any shock and/or vibration often causes the ball-and-socket joint 13 of the tension bar 12 to disengage from the circular hole 15 of the slider, which results in an error in the measuring of relative movement between both scales.
Further, when the stationary link 9 on which the slider 14 is mounted is removed from the detecting head 8 to separate the slider and main scale 6 from each other, the slider 14 can be freely and pivotally moved at the free end of the tension bar 12. However, it is almost impossible to contact the slider 14 with the main scale 6 again at a normal state to mount the stationary link 9 on the detecting head 8, because the slider 14 has an excessive degree of freedom, which results in the assemblies being troublesome.
Furthermore, the conventional linear scale measuring device has other disadvantages. The manufacturing process is complicated and the manufacturing cost is higher, because it requires the steps of covering the circular hole 15 with a cap 15b after the steps of providing the ball-and-socket joint 13 at the free end of the tension bar 12 and fitting it in the circular hole 15 of the slider 14.